M. tuberculosis and M. bovis are important pathogens of man and animals. M. tuberculosis is thought to infect up to a third of the world's human population, remaining undetected during a latent phase of infection and reactivating to cause 10 million cases of tuberculosis and other diseases per year resulting in 2 million deaths (Corbett et al., 2003). M. bovis, which has more than 99.9% sequence identity with M. tuberculosis, is the causative agent of bovine tuberculosis (BTB) and also causes disease in human. BTB represents a significant economic burden to the agricultural industries of various countries including the United Kingdom (Krebs, 1997; DEFRA, 2006).
Current methods of control for these mycobacterial infections centre on the live attenuated vaccine M. bovis Bacillus Calmette-Guerin (BCG) and diagnosis using an intradermal skin test with a purified protein derivative (PPD, tuberculin) harvested from mycobacterial cultures. The PPD skin test relies on a cellular immune response which is mounted in cattle with a mycobacterial infection. BTB control measures as applied for example in the United Kingdom and other European countries comprise a “test and slaughter” strategy where a positive result to a routine skin test with the single intradermal comparative tuberculin test (SICTT), leads to mandatory slaughter. In human populations the BCG vaccine has been used. However, BCG vaccination programs are hampered by widely differing rates of protection in different populations with efficacies that range from 0 to 80% (Colditz et al., 1994; Fine, 1995). In addition, vaccination sensitises individuals to tuberculin thereby interfering with diagnosis.
In addition to BTB skin tests, blood-based diagnostic assays that measure antigen-induced lymphokine production such as the interferon gamma (IFN-γ) are also under consideration. The cytokine IFN-γ appears to be critical in the development of immunity to M. tuberculosis. For example, both mice with a disrupted IFN-γ gene and humans with mutated IFN-γ receptor are highly susceptible to mycobacterial infections. However, specificity constraints are associated with the use of PPD in such assays. These arise due to the crude mixture of M. bovis proteins that PPD contains, many of which are cross-reactive with the BCG vaccine strain and environmental mycobacterial species such as M. avium and M. intracellulare. 
Previous studies have demonstrated that diagnostic reagents which distinguish between vaccinated and infected cattle can be developed using specific, defined antigens that are present in virulent M. bovis but absent from the BCG. Genetic analysis of BCG has revealed that several large genomic regions have been deleted during attenuation and subsequent prolonged propagation in culture. These regions have been characterised, and antigens from one of these regions, RD1, have been studied extensively in several species including humans and cattle. For example, it has been demonstrated that protein or peptide cocktails composed of two RD1 region antigens, ESAT-6 and CFP-10, can be used to distinguish between M. bovis infected and BCG-vaccinated cattle. The ESAT-6/CFP-10 assay is reported to have a sensitivity of at least 77.9% in cattle with confirmed tuberculosis, and a specificity of 100% in BCG-vaccinated and non-vaccinated cattle (Vordermeier et al. 2001).
However, the level of sensitivity achieved with these antigens has not reached that of tuberculin. It would, therefore, be desirable to provide other antigens in order to achieve this desired sensitivity. The present invention accordingly addresses the problem of providing further discriminatory diagnostic reagents for the detection of mycobacterial infections.
Camus et al. (Microbiology (2002) 148, 2967-2973) and the associated NCBI Accession no. NP_218132 is a disclosure of the genome sequence ofM. tuberculosis H37Rv, including the gene encoding Rv3615c. There is no suggestion of the use of the Rv3615c polypeptide or portions of it within a reagent for use in detection of M. bovis or M. tuberculosis infection in an animal.
Gamier et al. (Proc. Natl. Acad. Sci. U.S.A. (2003) 100, 7877-7882) and the associated NCBI Accession no. NP 857284 is a disclosure of the genome sequence of M. bovis, including the gene encoding Mb3645c. There is no suggestion of the use of the Mb3645c polypeptide or portions of it within a reagent for use in detection of M. bovis or M tuberculosis infection in an animal.
US2003/0129601 discloses a comparison of the genome sequences of M. tuberculosis and M. leprae and reports a total of 644 common protein sequences. It is proposed that these sequences may have a variety of uses including potential as drug targets, diagnostic antigens or subunit vaccine compositions. The inventors for the present application have found that one of the sequences has particular efficacy in the diagnosis of M. bovis or M. tuberculosis infection.